The present invention relates to a method for producing a cordierite body.
Cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) has excellent low thermal expansion properties. Therefore, cordierite is used as one material for parts requiring thermal shock resistance in order to endure the repeated cycles of rapid increases and decreases in temperature, such as a catalyst support for purifying exhaust gases.
The coefficient of thermal expansion of cordierite has been generally known as 26.0.times.10.sup.-7 /.degree.C. in the range of 25.degree..about.1000.degree. C. Recently, ceramic having an improved thermal shock resistance has been required. Upon making various studies, a cordierite body having an excellent low expansion property as low as 17.0.times.10.sup.-7 /.degree.C. (25.degree..about.1000.degree. C.) can be obtained by eliminating alkali metals, such as sodium and potassium, alkaline-earth metals, such as calcium and other impurities having a bad effect on the low thermal expansion property of cordierite, from the material.
Furthermore, it has been reported that by orienting the anisotropic property of the cordierite crystals, cordierite bodies having coefficients of thermal expansion of less than 11.0.times.10.sup.-7 /.degree.C. in the range of 25.degree..about.1000.degree. C. in at least one direction can be obtained.
For example, one of the methods for orienting the cordierite crystals to reduce the thermal expansion of the cordierite body in at least one direction is shown in U.S. Pat. No. 3,885,977.
In U.S. Pat. No. 3,885,977, there is stated that a cordierite body having thermal expansion as low as 11.0.times.10.sup.-7 /.degree.C. in a specific direction is obtained by forming a batch raw material containing plate-shaped clay particles or stacked clay which can be delaminated into plate-shaped particles during processing by such means as to impart a planar orientation to the plate-shaped clay particles, for example extrusion forming, drying and firing the obtained formed body.
As described above, by anisostatically forming a batch raw material mainly composed of plate-shaped particles of kaolin mineral and talc, a planar orientation is imparted to the plate-shaped particles, and the obtained cordierite body exhibits a low expansion property in the direction along the extrusion direction.
However, coefficients of thermal expansion of the obtained cordierite body in the direction perpendicular to the extrusion direction and that of the thickness thereof remain larger than that in the extrusion direction thereof. The difference of the coefficients of thermal expansion becomes larger as the coefficient of thermal expansion in the extrusion direction becomes smaller.
When coefficients of thermal expansion in the above described directions are largely different from each other, the cordierite body is liable to be damaged due to thermal distortion when being subjected to a rapid increase in temperature.
Therefore, when a honeycomb structured catalyst support for purifying exhaust gas is made of such a cordierite body as described above, the catalyst support is in danger of being broken due to the thermal distortion caused by repeated cycles of rapid increase and decrease in temperature, even if the wall thickness thereof is made thin.
Accordingly, one object of the present invention is to provide a method for producing a cordierite body having a low thermal expansion coefficient in any direction thereof.
Another object of the present invention is to provide a method for producing a cordierite body having excellent mechanical strength.
Still another object of the present invention is to provide a method for producing a cordierite body having excellent thermal shock resistance which is suited to be used as a catalyst support for purifying exhaust gas of a vehicle.